Today, the use of e-mails is widely spread. When the sender forwards an e-mail to a recipient, the action is immediate and unless a problem is encountered between the sender server and the recipient server, the e-mail is delivered in the recipient mailbox without any control on the way taken by the forwarded message in terms of network infrastructure.
Most countries have specific legal protections that prevent authorities and individuals from opening and reading the paper mail. Unfortunately, few countries have provided the same protections for the electronic mail, which gives individuals, companies and authorities a legal room to read the e-mails. Thus, the e-mails can be read at any of the routers along the path taken by the e-mail to reach its destination over the Internet. However, due to the growth of commercial and private contracts materialized by the electronic mail, it becomes crucial to be able to guarantee privacy of such exchanges.
To prevent attacks of e-mails, the usage of encryption algorithms either symmetric or asymmetric to secure the e-mail exchange over the Internet is widely spread. Thus, in the key encryption, there is a private key kept private for the owner, which is used to sign the data whereas a public key which can be known of many people is used for decrypting the message. To improve the security, such keys have a minimum of 40 bits but are longer and longer. For example, the symmetric algorithm Data Encryption Standard specifies 56-bit keys in some countries and 128-bit keys in other ones like the USA. Therefore, there is no doubt that such a continuous growth of the key length is not a solution for the security problem.